In recent years, secondary batteries such as lithium secondary batteries and nickel hydrogen batteries have become increasingly important as power supplies for mounting in vehicles using electricity as a driving source or as power supplies installed in personal computers, portable terminals and other electrical products. In particular, lithium secondary batteries, which are light weight and allowing the obtaining of high energy density, are expected to be preferably used as high-output power supplies for vehicle mounting.
In a typical configuration of this type of secondary battery, electrodes (positive electrode and negative electrode), which are provided with electrode active material layers (and more specifically, a positive electrode active material layer and a negative electrode active material layer) able to reversibly occlude and release a chemical species capable of functioning as a charge carrier on a surface of an electrode current collector, are laminated with a separator interposed there between. The separator that is interposed between the electrodes is a constituent material of the battery that prevents short-circuiting between both of the electrodes and is able to function as a conductive path (electrically conductive pathway) by being impregnated with an electrolyte.
For example, a porous sheet composed of a synthetic resin in the manner of a polyolefin-based thermoplastic resin is used as a separator of lithium secondary batteries. In addition, a separator is known in which an insulating layer is formed on a surface of the porous sheet to prevent the resin porous sheet from being damaged by heat when the battery generates an abnormally high amount of heat or to prevent the occurrence of short-circuiting due to the occurrence of lithium metal deposition (dendrites).
This insulating layer is formed by coating a paste prepared by mixing a solid material containing an insulating granular material with a suitable solvent (including that in the form of a slurry, and to apply similarly hereinafter) onto a surface of the above-mentioned porous sheet. An aqueous solvent may be used for the solvent that is mixed when preparing the paste. Pastes obtained by using an aqueous solvent have the advantage of producing lower levels of industrial waste accompanying use of organic solvents in comparison with pastes obtained by using organic solvents, while also not increasing equipment and treatment costs as a result thereof, thereby resulting in an overall decrease in the burden on the environment.
Examples of the prior art relating to this type of separator are described in Patent Documents 1 to 3. The technology described in Patent Document 1 suppresses internal short-circuiting caused by dendrite formation by constructing a secondary battery provided with a metal oxide film having a multilayered structure in a portion of a separator. The metal oxide film is formed by mixing a sol solution, in which fine particles of metal oxide are dispersed in a colloidal state in a solvent such as water or alcohol, with an amphiphilic dispersion of an ammonium compound and the like, spreading the resulting dispersion onto a separator and drying. In addition, Patent Document 2 discloses a technology for forming an insulating particle layer by coating a paste containing insulating particles mixed with a solvent such as water or alcohol onto a surface of an insulating base material, followed by drying and peeling off the coated film. Moreover, in Patent Document 3, a technology is disclosed in which a ceramic coating is provided on a non-woven fabric composed of non-conductive polymer fibers, and the ceramic coating is formed by coating a suspension containing a non-conductive oxide having for constituent elements thereof. Al, Zr and/or Si onto a base material.
Patent Document 1: Japanese Patent Application Laid-open No. H6-196199
Patent Document 2: Japanese Patent Application Laid-open No. 2005-276503
Patent Document 3: Japanese Patent Application Laid-open No. 2005-536857